Gravure printing is a type of intaglio printing in which the ink within the grooves in a printing plate is pulled out and transferred onto paper under pressure. Because it offers excellent shading repeatability, gravure printing is used in commercial printing for the production of magazines, catalogues, brochures, and others. The metal-plate cylinders used in gravure printing are harder than those used in offset printing. However, if the surface of the cylinder doesn't adhere to the paper fully during printing, the ink will not transfer properly and white dots called “missing dots” will result. The generation of numerous missing dots leads to lower printing quality.
The use of a coated paper offering greater smoothness and cushioning is said to be an important factor in suppressing the generation of speckles. A smoother paper adheres more closely to the cylinder, while a higher degree of cushioning allows the paper to deform under printing pressure, thus ensuring better adhesion to the cylinder.
The addition of coating pigments having high aspect ratios (such as delaminated clay and talc) is considered effective as a means of achieving smoothness in a coated paper for gravure printing. However, adding large amounts of high-aspect-ratio pigments in the coating-layer composition increases the viscosity of the coating mixture, which in turn makes mixture preparation difficult. As a result, streaks, scratches and other coating defects tend to occur. This limits the degree to which the solid content of the coating material can be increased, and as a result it becomes necessary to enhance the drying condition, which increases the cost while affecting paper quality by reducing sheet gloss.
The use of organic pigments, such as plastic pigments, is another method known to increase sheet gloss, prevent missing dots and add favorable characteristics such as opacity (Japanese Patent Application Laid-open No. 64-20396). However, organic pigments are more expensive than inorganic pigments, resulting in higher costs. Also, organic pigments tend to make the coating mixture more viscous under high shearing force, just as high-aspect-ratio pigments do, and this high viscosity makes the paper more prone to streaks, scratches and other coating defects.
To improve the cushioning property of a coated paper for gravure printing, generally the mechanical pulp content is maximized. Further, when chemical pulps must be used, varieties containing soft fibers are selected. However, given the increased environmental awareness throughout the public and industry, the use of recycled, deinking pulp is now favored over virgin pulp in both mechanical and chemical pulp applications. Therefore, at present it is somewhat difficult to obtain virgin mechanical pulps offering a sufficient amount of cushioning. With chemical pulp it has also become difficult to selectively source high-grade wood materials from which flexible fibers can be obtained, or to procure chemical pulp made from such high-grade wood materials.
Given the background explained above, there has been a need to develop coated gravure papers that provide favorable gravure printability in a manner not dependent upon the types and blending ratios of coating pigments and pulps used.